DE 199 18 552 A describes a harvesting machine with a harvested crop take-up arrangement, that is equipped with an adjustable hold-down arrangement for the harvested crop that limits its flow in the upward direction. In case of a harvested crop jam that is detected by a crop jam sensor, the hold-down arrangement is raised automatically in order to simplify the ejection of the jammed harvested crop during the reverse operation. The jam sensor detects the drive torque of one of the supply rolls or of the chopper drum of a forage harvester. In another embodiment, the jam sensor measures the torque on the shaft of the upper roll of the slope conveyor of a combine. The jam sensors described can detect jams within the harvesting machine, but are not arranged to detect jams in the front harvesting attachment.
In the subsequently published European patent application EP 02100450.2, a front harvesting attachment is described with a torque sensor inserted into the driveline of its crop conveying elements. The torque sensor transmits a torque signal that is conducted to a control arrangement which compares it with an allowable maximum value. If the torque signal exceeds the maximum value, the drive of the front harvesting attachment is automatically turned off. Here, as in the case of the harvesting machine according to DE 199 18 552 A, an additional sensor is provided that must be connected electrically with the harvesting machine. Therefore, a certain expense is required in order to be able to recognize a crop jam.
From U.S. Pat. No. 5,070,682 A, an acoustic detector has become known for sensing of an airborne noise that is expected to detect stones intruding into the crop supply mechanism of a forage harvester. This detector is not appropriate for the detection of a crop jam.